1. Field of the Invention
The present invention relates to positioners that control the valve openings of regulating valves. For example, the present invention relates to a positioner for an emergency valve that fully opens or closes urgently for security reasons when an abnormality occurs.
2. Description of the Related Art
Known examples of regulating valves used in process control of flow rates in, for example, plants include a regulating valve (so-called control valve) the valve opening of which is controlled by performing proportional-integral-derivative (PID) control such that the pressure of a fluid to be controlled is adjusted to a designated target value, and also include an emergency valve that fully opens or closes urgently for security reasons when an abnormality occurs (see JIS B 0100:2013).
Generally, an emergency valve is fixed in a fully open state or a fully closed state during normal flow control, and fully closes or opens rapidly if an abnormality occurs. In other words, an emergency valve does not operate unless there is an emergency. Therefore, it is necessary to perform a test on a regular basis for checking whether or not the emergency valve operates properly if there is an emergency.
International Publication No. WO 2015/171843 A2 discloses a partial stroke test (PST) as a known test performed on emergency valves. A PST is a method of inspecting whether or not there is a failure caused by fixation of an emergency valve by partially opening or closing the emergency valve instead of wholly operating (fully closing or opening) the valve. With this method, the emergency valve can be inspected without having to shut down the plant.
Generally, the valve opening of a regulating valve, such as a control valve or an emergency valve, is controlled by a positioner. The positioner calculates a deviation between a target valve-opening value designated by a higher-level device and an actually-measured valve-opening value (actual degree of opening) of the regulating valve and supplies a control signal generated based on the deviation to an operational unit for operating the opening and closing of the regulating valve so as to control the valve opening of the regulating valve.
A positioner for a control valve performs feedback control (PID control) so as to keep the valve opening of the control valve constant. In contrast, a positioner for an emergency valve performs open loop control so as to open and close the valve when an emergency occurs. For example, in the case of a positioner for an emergency shutoff valve, the positioner performs open loop control to set the target valve-opening value to 100% and fully open the valve during normal operation, and to change the target valve-opening value from 100% to 0% and rapidly close the valve when an emergency occurs. If a PST is to be performed, the valve opening is controlled by performing feedback control (PID control) even in the case of a positioner for an emergency shutoff valve.
A positioner is equipped with an electro-pneumatic converter (I/P converter) including a nozzle flapper that converts an electric signal MV based on a set valve-opening value designated by a higher-level device into a pneumatic signal (pressure Pn) and a pilot relay unit that generates an output pneumatic signal (output air pressure Po) by amplifying the pressure of the pneumatic signal output from the nozzle flapper. In the electro-pneumatic converter, the pilot relay unit has a small deviation from a designed value of the input-output characteristic (Pn-Po characteristic) due to, for example, aging degradation or a temperature change, whereas the nozzle flapper has a large deviation from a designed value of the input-output characteristic (MV-Pn characteristic) due to, for example, aging degradation, the ambient temperature, or clogging of the nozzle outlet. Thus, the input-output characteristic (MV-Po characteristic) of the entire electro-pneumatic converter changes significantly due to, for example, aging degradation. This results in a significant change in the input value (electric signal MV) to the electro-pneumatic converter when the pressure (output air pressure Po) of the pneumatic signal to be output from the electro-pneumatic converter starts to change from the maximum value (or the minimum value).
The input value (electric signal MV) to the electro-pneumatic converter when the output air pressure Po of the electro-pneumatic converter starts to change from the maximum or minimum state will be referred to as “operating point” hereinafter.
In the case of a control valve, since the positioner performs PID control so as to match the actually-measured valve-opening value with the target value, as described above, the above-described problem with regard to fluctuations in the operating point does not occur.
In the case of an emergency valve, since the positioner performs open loop control to control the valve opening, if the operating point fluctuates, there is a possibility that the emergency valve may malfunction during normal operation. Therefore, emergency valves are designed in view of a sufficient margin for preventing them from malfunctioning even if the operating point fluctuates. For example, emergency shutoff valves are designed such that the output air pressure is at maximum (i.e., actual degree of opening of 100%) when the input (electric signal MV) to the electro-pneumatic converter is 60% or higher even if the input to the electro-pneumatic converter is not 100%, as shown in FIG. 13.